PART VI.

Temperature best adapted for working Cylin-drical Retorts.

There is perhaps no subject in the art of manu-facturing coal gas, on which practical men are lessagreed, than they are on the temperature mosteconomically to he employed for the production ofcoal gas in the large way. It must be sufficientlyevident, that cast-iron retorts, when worked at alow temperature, will last longer, than when ex-posed to higher degrees of heat.^ Hence, accor-ding to some operators, the economy of the processconsists in saving- the retorts, at the expense of a

* It is essential that the retorts should be kept in constant actionnight and day, or at least never allowed to go below a red heat. Thefirst portion of oxide which forms upon the surface, when allowed tocool, cracks and falls off, leaving a new surface to be acted upon thenext time it is heated. By thus being every day heated and cooled, aretort will be speedily destroyed.